The invention relates to a new method of dimerizing isophoronediisocyanate with triazolate catalysts, the resulting products and starting components for polyurethane plastics and coating powders containing these products.
The production of polyisocyanates with a uretdione structure by catalytic dimerization and optionally simultaneous trimerization of monomeric aliphatic or cycloaliphatic diisocyanates is known. A comprehensive summary of the industrially relevant state of the art dimerizing methods and the catalysts or catalyst systems used in the process can be found in J. prakt. Chem. 336 (1994) 185-200.
Among the light-resistant uretdione polyisocyanates the linear dimerized forms of 1-isocyanate-3,3,5-trimethyl-5-isocyanate methylcyclohexane (isophoronediisocyanate; IPDI), without isocyanurate groups, have special properties. They may for example act as preferred starting components in the production of cross-linking agents for polyurethane (PUR) coating powders not containing any blocking agents (as described for example in EP-A 45 996, EP-A 639 598 or EP-A 669 353). However, uretdione polyisocyanates obtained from IPDI by most of the known processes have a very high or fairly high trimer content, dependent on the type of catalyst used. At the present time only two catalyst systems are known, which can be used to obtain virtually linear IPDI dimers such as are required as starting components for uretdione cross-linking agents for coating powders.
EP-A 45 995 describes the use of special peralkylated aminophosphines as catalysts for selective dimerization of IPDI. The dimerization catalyst preferably used, hexamethylphosphorous acid triamide (tris-(dimethylamino)-phosphine), enables pure IPDI uretdiones to be prepared with a trimer content less than 2 wt. %. The serious disadvantage of the method is the high sensitivity of aminophosphines to oxidation. This causes for example hexamethylphosphorous acid triamide to react spontaneously in the presence of atmospheric oxygen to form hexamethylphosphoric acid triamide (HMPT), a compound known to be highly carcinogenic. This side reaction, which cannot be totally excluded on an industrial scale, greatly restricts practical application for workplace health and safety reasons.
EP-A 317 744 describes a method of preparing linear (cyclo)aliphatic uretdiones by catalysis with 4-dialkylaminopyridines such as 4-dimethylaminopyridine (DMAP). This method also provides linear IPDI uretdiones almost free of isocyanurate groups, although these have a very strong inherent color. By using phosphine or phosphite stabilizers in the DMAP-catalyzed dimerization in accordance with EP-A 735 027 or EP-A 896 973 the color quality can indeed be improved, but the improvement is often still not sufficient for special applications, for example to prepare uretdione cross-linking agents for clear polyurethane coating powders.
The two catalyst systems, the aminophosphines of EP-A 45 995 and the dialkylaminopyridinees of EP-A 317 744, both have the additional disadvantage of having only moderate catalytic activity with IPDI. High catalyst concentrations of up to 2 wt. % have to be used to obtain a fast enough reaction.
It is an object of the invention to provide a new method of preparing uretdione polyisocyanates which produces mostly linear, light-colored products, preferably free of isocyanurate, using highly reactive and selective catalysts.
The object was achieved by preparing them by the method of the invention.
The invention relates to a method for preparing a polyisocyanate with at least one uretdione group by oligomerizing part of the isocyanate groups of isophorone diisocyanate in the presence of a catalyst which accelerates the dimerization of isocyanate groups, wherein the oligomerization catalyst contains a saline compound containing from 10 to 100 wt. % of 1,2,3- and/or 1,2,4-triazolate structures (calculated as C2N3; molecular weight=66) in the anion.
The subject of the invention is also the use of the uretdione polyisocyanates made by this method as starting components for polyurethane plastics, particularly as isocyanate components for producing uretdione cross-linking agents for coating powders.